Uptake of maternal immunoglobulin G (IgG) by the neonatal Fc receptor (FcRn) provides newborn mammals with a passively acquired humoral immunity against environmental antigens encountered by the mother. In neonatal rodents, FcRn expressed on the luminal side of intestinal epithelial cells binds the Fc portion of maternal IgG from ingested milk. The IgG/FcRn complex is transported across the cell and released into the slightly alkaline bloodstream of the neonate. The complex of FcRn with the Fc portion of an IgG has been solved to low resolution, ~6_, and published (Burmeister, et. al. Nature (1994) 372, pp 379-383 ). In an attempt to extend the resolution of this structure, we have prepared complexes of FcRn with various monoclonal Fc preparations, and obtained single crystals. Unfortunately these crystals did not diffract better than our published data in recent experiments at CHESS. The interaction of Fc with FcRn is strongly pH dependent (Kd 10 nM at pH 6.0 and undetectable at pHs greater than ~7.5). In order to understand the structural basis of this pH dependence, we have recently obtained crystals of FcRn at pH 8.5 that diffract to ~4_ at our home source and beyond 2.5_ at CHESS. Unfortunately these crystals, even though they are cryopreserved, suffer radiation damage. We were able to combine data from several crystals to obtain a data set with an effective resolution of ~2.9 _ and an r-merge of ~9%. We anticipate that we additional beam time we can collect a complete data set to about 2.5 _.